well.. keeping your nitrate at 0 is almost impossible...
So, you should get some level eventually anyway 5-20 ppm
depending on how often, how much water you change and how much food you put into the tank.
If you were getting these reading I would suggest that you may have been having trouble using your testing kit.
If not and your nitrate is continuing to increase, find out whats rotting, find out if someone else is feeding and get them to stop, feed less, increase the percentage and or freqency of your water changes.. To add to this, make sure your filter doesnt have muck in it.